The present invention relates to a process for preparing pure neopentyl glycol.
The production of neopentyl glycol (NPG) is accomplished conventionally by reacting isobutyraldehyde with formaldehyde in the presence of alkaline compounds, such as alkaline solutions, alkali carbonates, tertiary amines, etc., as the catalyst. While such syntheses can be conducted without difficulty, the purification of the thus-produced NPG, contaminated by esters, is expensive from a technical and industrial viewpoint. Although processes are known wherein a relatively pure crude product is obtained, such as, for example, in processes using tertiary amines as the catalysts or operating with the addition of water-soluble alcohols in a homogeneous phase (see "Ullmanns Enzyklopaedie der technischen Chemie" [Ullmann's Encyclopedia of Technical Chemistry] 7 [1973]: 228; exact analytical data have not been divulged), these modes of operation attain their advantage of increased purity of the crude product at the cost of a considerable additional expenditure during the synthesis. The tertiary amine catalysts must first be recovered in two additional distillation stages--the first directly after the synthesis and the second stage after the hydrogenation (during which the formates are cleaved). Furthermore, an especially careful distillatory purification of the reaction products is necessary to maintain the nitrogen content thereof at a minimum level.
When a water-soluble alcohol, e.g. methanol, is used in the synthesis, its presence makes it impossible to separate the waste-water stemming from the aqueous formalin solution and the alkaline solution and/or alkali carbonate solution. For this reason, methanol and unreacted isobutyraldehyde are distilled off in additional process stages; the residue of the distillation is extracted with n-dibutyl ether; the extract is washed with water and hydrogenated; and thereafter the ether is separated by distillation.
U.S. Pat. No. 3,939,216 teaches that the reaction of isobutyraldehyde with formaldehyde can be conducted at very low expense without any problems in the presence of aqueous soda solution. However, the crude product obtained in this process contains 10-15% ester, namely, not only the hydroxypivalic acid-NPG ester, readily separable from NPG by distillation, but also--and this is not disclosed--the monoisobutyric acid-NPG ester.
This ester cannot be separated by distillation from NPG at an industrially tolerable expense. Since, heretofore, it has been possible only under extreme conditions, during which NPG is partially decomposed, to hydrogenate this ester to NPG and isobutanol, this ester constitutes the essential cause for the difficulties encountered in purifying and manufacturing NPG.
In the process described in this patent, the esters are removed by the following method:
After the hydrogenation, the esters are saponified with sodium hydroxide solution and the product is subsequently subjected to steam distillation under vacuum. This is done under maximally mild conditions at temperatures of below 140.degree. C. and with a short residence time with the aid of a thin-film evaporator to extensively avoid NPG losses. To maintain the salts obtained in the thin-film evaporator in the fluid state, the NPG content in these salts must not drop below a certain value. Otherwise, plugging problems are encountered in the thin-film evaporator which can only be eliminated after shutdown of the apparatus by flushing with water. Due to such shutdown periods, the production capacity of the thin-film evaporator is lessened by 10-15%. For this reason, a certain quantity of NPG is allowed to remain in the waste salts, and the latter are worked up in a separate process stage. At the head of the thin-film evaporator, a dilute aqueous NPG solution is obtained from which the water is separated in a distillation stage, which consumes a large amount of energy.
Inasmuch as this mode of operation is very expensive, many attempts have been made to hydrogenate ester-containing NPG directly to pure NPG. DOS [German Unexamined Laid-Open Application] 1,804,984 (=British Pat. No. 1,219,162) proposes, as the hydrogenation catalysts, barium-activated copper chromite catalysts and temperatures of 175.degree.-220.degree. C.
It has been taught that hydroxypivalaldehyde can be readily hydrogenated to NPG at temperatures of below 160.degree. C., but that temperatures of 200.degree.-210.degree. C. are required to reduce the esters present as impurities (British Pat. No. 1,219,162). In this process, a product was utilized which contained a small amount of ester since its manufacture was conducted with the addition of methanol (see above). Yet, these high temperatures are required to obtain low ester contents of 0.1%. The relatively low and fluctuating yields of less than 80% demonstrate that NPG was partially decomposed at the high hydrogenation temperatures.
DAS No. 2,054,601 (=U.S. Pat. No. 4,094,914) generally states that the hydrogenation of hydroaldehydes in the liquid phase is accompanied by the formation of undesirable by-products, due to long residence times required to complete the reaction, whereby the yield and purity of the diols are impaired. The effect is further aggravated by the residual alkali contents stemming from the synthesis. For this reason, this reference suggests that the hydrogenation be conducted in the gaseous phase. Here, using a nickel-containing catalyst at 128.degree. C., a selectivity of between 98% and 99% is attained. However, the product used as the starting compound does not contain the monoisobutyric acid ester of NPG (abbreviated: MB-NPG). Therefore, this disclosure does not set forth whether, by means of this process, it is possible to solve the above-described, primary problem in NPG manufacture. Furthermore, a disadvantage inherent in this process is the high energy consumption because both the product utilized as the starting material and the required solvent must be vaporized.
All conventional processes, therefore, exhibit considerable disadvantages, in that they either avoid the formation of MB-NPG in the synthesis by special, expensive measures, or they remove this ester by alkaline saponification and then must keep the damaging effect of the sodium salts at a minimum by expensive technical, industrial installations.